The present invention relates to a traveling wave tube which includes a helix type delay line circuit having a velocity taper for suppression of the backward traveling wave oscillation.
As is well known in the art, a traveling wave tube is constructed with an electron gun section for emitting an electron beam. A delay line section causes interactions between the electron beam and electromagnetic waves. A collector section collects the electrons which have finished their interactions with the electromagnetic waves. An electromagnetic wave output section guides the electromagnetic waves into the delay line circuit section. Among sections, the delay line section reduces the phase velocity of the electromagnetic waves, coming from an electromagnetic input section, to a low level which is substantially the same as the velocity of the electron beam, while maintaining the synchronous relationship between the electron beam and the electromagnetic wave. There is an amplifying action. Delay line circuits are constructionally classified into several types including a helix type delay line circuit and a coupled cavity type delay line circuit. The present invention is directed to helix type delay line circuits.
Because of constructional simplicity in comparison with the coupled cavity or other delay line circuits, the helix type delay line circuit has conventionally been used widely as the delay line circuit of a high frequency amplifying tube, for relatively low or intermediate power. However, the helix type delay line circuit is thermally weak because it is constructed so that a helix or a thin wire coil is supported by means of dielectric rods having a low thermal conductivity. The helix type delay line circuit produces a backward wave oscillation if it is operated in a high frequency and with a high power. Therefore, it has not been used as a high frequency and power amplifying tube. However, the thermal problems of the helix type traveling wave tube have recently been solved, with the progress of the manufacturing technique. At the same time, it becomes necessary to suppress the oscillations of the backward traveling waves.
According to an analysis of the electromagnetic field of a helix, it can propagate many space harmonics as well as the fundamental waves which have positive phase and group velocities. The fundamental waves, not harmonics, are practically used for amplification. The backward traveling oscillating waves constitute the problem in the helix type traveling wave tube, are caused by the interactions between the electron beam and the backward traveling component of the minus-1 space harmonic having a positive phase velocity and a negative group velocity. This minus-1 space harmonic component is increased with the increase in free space with the phase constant ka (where Ka=.omega.a/c, wherein:.omega. stands for the angular frequency of the electromagnetic waves; c stands for the velocity of light; and a stands for the average radius of the helix). In the high frequency and power type of helix-type traveling wave tube, on the other hand, the phase constant is .beta.a (where .beta.a=.omega.a/v.sub.p, wherein:.omega. stands for the angular frequency of the electromagnetic waves; v.sub.p stands for the phase velocity of the electromagnetic waves; and a stands for the average radius of the helix). The electromagnetic waves propagated on the helix have to be selected with a value within a preset range (1 to 2). Since the operating voltage has to be increased by the requirement for the output power and the beam focus, the value of the phase constant ka is also increased so that the backward traveling waves become liable to oscillate.
For suppressing the oscillations of the backward waves, two methods have conventionally been proposed. One is the method published in the International Electron Device Meeting (IEDM) in December, 1978 by the paper (pp526-529) which employs dielectric rods for supporting the helix, which is baked with an electromagnetic wave absorbing material having a meandering line shape to selectively attenuate a frequency which is around the frequency at which the backward traveling wave oscillation occurs. This attenuation suppresses the backward wave oscillations. This first method can produce a tube having a satisfactory operating characteristics. Not only the frequency of the backward wave oscillation but also all of the frequencies other than those in the used band of the fundamental waves can be considerably attenuated. However, the technique of baking the dielectric rods with the electromagnetic wave absorbing material having the meandering shape is so difficult that the first method impractical.
A second method is disclosed in U.S. Pat. No. 3,761,760 issued Sept. 25, 1973 to R. Harper et al, which provides an output-side delay line circuit with a special velocity taper. This second method is based on the concept that the energy of the electromagnetic waves is absorbed by the electron beam if there are interactions between the fast space charge waves of the electron beam and the electromagnetic waves. Therefore, the backward traveling wave oscillation can be suppressed if the oscillation frequency of the backward traveling waves and the fast space charge waves are coupled at an electromagnetic wave attenuator on the output-side of a delay line circuit.
According to this second method, however, it is necessary to provide a velocity taper at a rate as high as about 20% to the phase velocity. Therefore, it has the disadvantages that the matching characteristics deteriorate for the fundamental waves. An additional length of the tube is necessary for the suppression of the backward traveling wave oscillation, which increases the length of the tube. Another disadvantage of the second method is that the plasma frequency and the oscillation frequency cannot be determined on principle when the helix pitch has an effect upon the synchronism between the fast space charge waves and the backward traveling waves. A further disadvantage of the second method is that the synchronism between the fast space charge waves and the backward traveling waves is really difficult because of the production accuracy of the helix size causes an operational instability.